The present invention relates to an improved skirt system for an air cushion vehicle. Such vehicles are supported by a pressurized air cushion contained within a depending skirt installed about the bottom periphery of the vehicle's hull. The skirt consists of a peripheral bag fixed to the hull and a bow seal, a stern seal, and side seals extending downward from the bag. The bag is generally inflated by pressurized air from the air cushion supply. Once inflated it acts as a fluid distribution medium and a resilient support for the vehicle to absorb and distribute shock. The seal assembly defines the air cushion containment structure and as such must be delicately balanced to provide pitch and roll stability, low drag, structural integrity, and restoring moment in addition to its basic role of air cushion containment.
Among the problems encountered with known skirt designs are (a) the tendency of the inflated seal at the bow of the vehicle to tuck under, deflate against the hull and cause the craft to plow in or pitch forward upon encountering rough seas; (b) the necessity for longitudinal and transverse stability seals and their associated ducting, air flow and maintenance requirements; (c) pitch and roll stability; (d) excessive drag; (e) payload restrictions depending upon speed and sea conditions; (f) cushion air leakage through the skirt elements; (g) skirt and seal damage on contact with obstacles; and (h) the quality of the ride over waves and rough surfaces.
In the most common configuration currently in use the seal elements are open loop shaped fingers depending downward from the bag and being open at its interior to the air cushion, as shown in U.S. Pat. No. 4,279,322 at FIG. 5. To enhance the stability of this design, longitudinal and are used as shown in U.S. Pat. No. 3,850,126. These seals increase the complexity of the system and create severe maintenance problems. It is one purpose of this invention to eliminate the need for longitudinal and transverse stability seals.
In another early competitive skirt design, the seal was constructed as a continuous assembly of contiguous downward tapered, conically shaped sleeves depending from the bag about the lower periphery of the vehicle. The sleeves define individual cells open at the bottom and communicating with the interior of the bag at the top to receive pressurized fluid therefrom. To provide rigidity to the otherwise flexible sleeve, each sleeve is pressurized by bleed air from the peripheral bag. As such seals dip into the water with the pitch or roll motion of the vehicle, there is a pressure increase in the sleeve cell which generates a restoring moment at that position to stabilize the vehicle. In some instances, the sleeve of the prior art is maintained at a pressure which is greater than the air cushion pressure and opens in a plane generally parallel to the terrain being traversed.
A differential restoring pressure occurs because of the movement of the water surface upward against the sleeve, thereby compressing the pressurizing fluid and extending the force over an increasing area because of the conical shape of the sleeve.
A serious problem occurs with the bow and side seals of the prior art when the vehicle encounters waves or when the vehicle pitches or rolls in the general direction in which the vehicle is travelling. This motion causes the sea to catch the seal with substantial force which may tear the seal and bag from the hull, thereby causing a complete failure of the seal structure. This immediately creates a breach to air cushion containment and a resultant serious instability condition. This generally occurs in surf or rough seas when stability of the vehicle is of prime concern.
Bow and side seals of the prior art are shown in FIGS. 1 and 2 respectively. In these systems the generally conically shaped sleeves have openings that are parallel to the plane of the terrain. This combined with an outward slant to the air cushion side face of the sleeve results in a scooping effect on rough water severely stressing the seal structure.
The seal system at the stern of the vehicle differ from the seals at the bow and sides of the vehicle since the stern sleeves, although open at the bottom, must be closed on the inboard side in the direction of the air cushion in order to provide a smooth planing surface which permits the vehicle to move freely over terrain without snagging or scooping. A conical stern seal design is shown in U.S. Pat. No. 4,834,011. Therefore, while such stern seals become pressurized, they only permit the escape of pressurizing air when they are lifted off the surface of the water. A stern seal of the prior art is shown in FIG. 3.
It is the purpose of this invention to provide a unique seal system which provides enhanced stability of the vehicle particularly in rough seas during which it is most important. The seal system of this invention is designed to avoid catastrophic breaching of the seal system by maintaining the structural integrity of the seal system under all conditions. In addition an improved and more responsive restoring moment is provided at all times to more efficiently compensate for pitch and roll motions. All these features are established with a decrease in overall drag.